The crack time is an estimation of the time needed for a hacker to guess your password. SafeInCloud uses Zxcvbn estimation algorithm developed by Dropbox. This algorithm validates a password for dictionary words (in English), sequences (e.g. 123 or abc), repeats (e.g. aaaaa), substitutions (e.g. 0 for o, 4 for A) and so on. Read more at Dropbox Tech Blog.
The basic rule of a good password: make it long (e.g. a phrase of 2-3 words).